Gang saws and respective sawing methods for the multiple lap-cutting of mono- and polycrystalline rods or blocks of a semiconductor material such as silicon, germanium, gallium arsenide, indium phosphide, and oxidic material such as sapphire, quartz or gallium-gadolinium garnet are known (cf. e.g. DE-PS No. 20 39 699, DE-OS No. 27 57 132 or DE-OS No. 27 22 782). In all these methods, due to the irregular wear caused by the different stresses at the central and peripheral regions of the blades as they cut into the workpiece, the blades acquire a concave contour which becomes more and more pronounced with increasing sawing time. At the reversal points, this results in ever greater impact which stresses the workpiece during the sawing process and can lead to chipping and in the extreme case, to destruction of the workpiece. Heretofore, several ways have been tried to eliminate this problem.
It is known practice to glue lateral wedges, e.g., of silicon, for example, on silicon blocks which, as starting material for solar cells, are sawed to form square disks of 10 cm side length and about 300 to 600 microns thick. One obtains thereby, a workpiece which tapers with progressing sawing. Since at constant blade excursion, the actual operating length of the blade shortens continuously and the reversal point moves steadily toward the center of the blade, blade impact can be avoided almost completely. This method, of course, requires an additional step for gluing on the wedges. The production of the wedges is expensive and complicated since they must be shaped to fit.
In addition to these methods which shorten the operating length of the blades at constant blade excursion, it is known that the blade excursion itself can be shortened to avoid blade impact. Heretofore, however, continuous shortening of blade excursion has been described only for the rarely used hydraulically driven gang saws (cf. DE-OS No. 27 57 132) or for gang saws oscillating in a swinging spring system (cf. DE-OS No. 27 39 257), where it is difficult to maintain a precisely defined reversal point. In the conventional gang saws driven through a rotating drive crankhead with a connecting rod system, in which the reversal point is always exactly fixed, the machine must be stopped from time to time to shorten the excursion and by laboriously displacing by hand the connection of the connecting rod system at the crankhead. The duration of the sawing process is lengthened and the risk of damage to the workpiece as well as to the blades is increased by a repeated, necessary and complicated start-up of the apparatus.